Many countries have, or are in the process of, imposing severe limits on the concentration of chlorinated hydrocarbon compounds permitted to be present in a wide range of materials used in industrial and other environments. Typical such compounds include polychlorinated biphenyls (PCBs), hexachlorobenzene, DDT and dioxins as well as hydrocarbon oils containing chlorinated aromatic compounds such as polychlorinated biphenyl compounds which are commonly present as one of the waste products in used or spent oils especially transformer oils and other similar liquids. Such materials are commonly destroyed by high temperature incineration, but this route is expensive and not permitted in some countries.
Because hydrocarbon transformer oils represent a large capital investment, any commercial process for the destruction of chlorinated organic compounds therein, and especially PCB contaminants in hydrocarbon transformer oils should, ideally, result in a product oil which can be reused in transformers. In principle, this could be accomplished by devising a process which destroys the PCBs, but which is carried out under reaction conditions which do not significantly alter the chemical composition of the hydrocarbon transformer oil.
Apart from combustion, either direct or catalysed, there are two main types of processes used to remove PCBs from oils. In one, the PCBs are reacted with sodium metal under specialised process conditions. Several variations of this route have been described (1-3). In general, this is a difficult process to scale up, requires routine handling of hazardous chemicals, and is likely to be operated as a batch process. It is uncertain if this approach can routinely result in a hydrocarbon transformer oil product which can be reused.
In another, (4-9) hydrocarbon oil containing the PCBs is reacted with hydrogen over a catalyst. In these processes, the hydrogen chloride formed in the course of the reaction moves through the reactor unchanged, and is washed from the reaction products only after these leave the reactor. These processes were exclusively designed to destroy the PCBs, and were not designed to recover an oil having the specific dielectric and other properties required for a high quality hydrocarbon transformer oil. Indeed, the coexistence of the hydrocarbon transformer oil and the hydrogen chloride gas in the catalytic reactor at high temperatures and pressures is detrimental to the stability of the catalyst and to the chemical composition of the hydrocarbon oil. Its presence, and the reactions the hydrogen chloride can undergo will render the oil unsuitable for subsequent use as hydrocarbon transformer oil.
Over time oils will gradually deteriorate due to oxidation of the hydrocarbons and contamination by other impurities. It is desirable to remove the oxidised species as the presence of these species can significantly reduce the electrical properties of the oil. It is also desirable to remove any organohalogen compounds as they may also affect the properties of oil and acid generated from them can cause a breakdown of the oil if the oil is hydrogenated by standard methods to remove the oxidised species. Particular examples of organohalogen compounds that may generate acids include polychlorinated biphenyls (PCBs), chlorinated napthalenes, chlorinated benzenes and halogenated solvents. Eventually, such degraded and contaminated oil must be withdrawn, as further use greatly reduces the efficiency of transformer operation and can ultimately lead to failure of the transformer. Such degraded oil is generally disposed as low grade fuel oil, valued at less than 1/4 its original cost. When the oil is hydrogenated by standard methods significant breakdown of the oil during hydrogenation by standard methods can also be caused by the catalyst itself, usually due to acidic sites on the catalyst support.
Oils such as hydrocarbon transformer oils represent a large capital investment. Accordingly, there is a need to develop a cheap, effective and robust method to regenerate the deteriorated oil. Ideally, the process results in a product oil which can be reused for its original use. For transformer oils, this means regenerating an oil which has electrical properties equivalent to the original oil. Ideally, the process would reverse the oxidation reactions by converting the oxidised species back into hydrocarbons, but which is carried out under reaction conditions which do not significantly alter the chemical composition of the oil.
The total transformer oil inventory in use throughout Australia is around 240 million liters. At a cost of about $1000/tonne, this material has a replacement value of approximately $214 million.
In the recent Australian report of the Independent Panel on Intractable Waste (1992) estimated that about 67,000 tonnes of this oil, valued at almost $60 million, is contaminated with PCBs (polychlorinated biphenyls). State and Federal Governments are currently developing legislation to restrict the further use, storage and disposal of transformer oil contaminated with toxic PCBs, and are committed to the removal of these materials from the environment. The treatment or disposal of this oil poses a serious problem for its owners in the light of the decision of the federal government to abandon plans for construction of a toxic waste incinerator in Australia, and also to ban export of PCB contaminated materials to overseas incinerators. These decisions virtually force the development of an indigenous technology for the destruction of such toxic materials.
The transformer oil inventory in Australia and elsewhere clearly constitutes a major resource within the electricity generating and distribution industries. At present, there is no process available which can economically regenerate the dielectric properties of degraded transformer oils and at the same time remove any PCB contamination.
Further, there is a need for a process capable of destroying halogenated hydrocarbons generally.